Many catalysts for ammoxidation of propylene to produce acrylonitrile are molybdenum containing poly-atomic catalysts. For example, the catalysts disclosed in U.S. Pat. No. 4,162,234 and Japanese Patent No. 58-2232 (corresponding to U.S. Pat. No. 4,228,098) are all molybdenum, bismuth and iron containing polycomponent catalysts. Chinese Patent CN86101301 discloses a molybdenum, bismuth and lead containing poly-component catalyst. Because the ammoxidation of propylene takes place at a temperature of 430.degree.-470.degree. C., and the volatilization of molybdenum component is unavoidable in any one of molybdenum-containing catalyst, thus the life-time of the catalyst is influenced. J. Buiten (J. Catal., 1968, 10(2), P. 188) provides a method to calculate the loss of molybdenum by means of the quantitative relation between the quantity of molybdenum volatilized and the temperature and the partial pressure of vapor.
Fluidized bed reactors are mostly used for producing acrylonitrile. Besides the influence of the volatilization of molybdenum, which makes the composition of the fluidized bed catalyst change, upon the activity of the catalyst, the contact efficiency between gas phase and solid phase also influences the optimum properties of the catalyst used in fluidized bed reactors. The amount of fine particles, especially the particles of less than 45 .mu.m contained in fluidized bed catalysts influences directly the contact efficiency between gas phase and solid phase. If the amount of fine particles contained in fluidized bed catalyst is too low, for example, lower than 10 percent by weight, the conversion of propylene to acrylonitrile would decrease significantly.
In order to solve the above-mentioned problems, many methods have been proposed to maintain the stable operation of the fluidized bed reactor for producing acrylonitrile. For example, Japanese Patent No. 63-30065 proposes a method that the catalyst in the fluidized bed reactor is discharged from the reactor after a period of operation, and the discharged catalyst is reintroduced into the reactor for operation after the compensation of the loss of molybdenum component of said fluidized bed catalyst. U.S. Pat. No. 3,882,159 proposes to add molybdenum oxide or inert particles containing molybdenum oxide into the reactor. U.S. Pat. No. 4,590,011 proposes a method that a partial catalyst is drawn out from the reactor after a period of operation, the catalyst drawn out is screened to remove the big particles and is then reintroduced into the reactor for operation; or fine inert particles are added into the reactor to compensate the loss of fine catalyst particles in said fluidized bed catalyst. Japanese Patent No. 63-30065 discloses a method that the fluidized bed catalyst used in a fluidized bed reactor for a long period is pulverized by means of pulverizer to increase the amount of fine particles in said fluidized bed catalyst.
The above-mentioned methods have some shortcomings. The methods of drawing out catalyst from the fluidized bed reactor and compensating the loss of molybdenum component, or increasing the amount of fine particles contained in the fluidized bed catalyst by means of screening and crushing have the shortcomings that the operation of the reactor must be stopped which will result in economic loss. In addition, apart from the inconvenience of the treatment of the drawn out catalyst, the catalyst particles after being crushed are no longer spherical particles, which will influence the fluidization in the reactor. When molybdenum oxide or the material containing molybdenum oxide is added into the reactor, although the loss of molybdenum can be compensated, but as the material added into the reactor is mixed mechanically with the original fluidized bed catalyst, therefore, more molybdenum oxide would flow out from the reactor due to the differences between their physical properties, which will result in the easy block-up of the following condensers, and said condensers must be purged regularly.